Great find. If you want to maintain its original look use soap, water, sand and a few pennies and place in a small container and shake it a bit. The dark color will stay but the coin will clean up pretty good. If you want it to original color put vinegar and some salt (the more salt to more corrosive the solution) with the nickel in a container and shake a bit and soak. The longer you keep in solution the more corrosive affect (15-20 minutes with some shaking should clean it up pretty good). When done a light SOS rub to your liking. Cleaning is cleaning so don't clean it if you want to keep it as original as possible. Nice job. WD